1. Field of the Invention
This invention relates to the field of microlithography, in particular to the printing of integrated circuit patterns with a feature size of less than 1 micron.
2. Prior Art
In the photo-lithographic fabrication of integrated circuits, films sensitive to radiant or particle energy are exposed in predetermined patterns to define circuit features. In some cases, the energy is passed through masks which define the patterns, thereby selectively exposing a photoresist film on a semiconductor body. In other instances, the film is on a mask substrate and the film is exposed as a step in the making of the mask. Other times the direction or pattern of the radiant energy itself is controlled to define patterns in the film. This can be done as part of making a mask or reticle or to directly "write" onto the photoresist film covering a semiconductor wafer.
As integrated circuits are developed that are more densely populated with devices, the feature size of the discrete devices within the integrated circuit become smaller. This is important in both the making of masks or reticles or writing onto the photoresist film covering a semiconductor wafer. To aid in increasing the density, the ability to print integrated circuit patterns with small feature sizes is necessary. The tool of choice for printing integrated circuit patterns with feature sizes between 0.5 and 1.5 microns is the reduction step and repeat camera, commonly known as a stepper. A step and repeat camera utilizes reduction optics to reduce a reticle image for writing directly onto a photoresist film covering a wafer. The writing occurs in a stepwise, i.e. move reticle, expose and repeat fashion across the wafer. The step and repeat camera has the drawbacks of being relatively slow and having a relatively small field size (limited by the design of the lens).
A second known technique for printing a circuit pattern on a wafer is through scanning. In a scanning system, a radiant energy source is projected through a mask and an arc-shaped aperture to create a well corrected image field. The field image is then directed towards a semiconductor wafer covered by a photoresist film. The reticle and semiconductor wafer are concurrently scanned through the arc-shaped object and image fields, so that the entire reticle image is printed onto the wafer. No reduction optics are utilized. Scanning systems generally are faster than step and repeat cameras, but are not suitable for printing wafers with small feature sizes.
As feature sizes have shrunk, die sizes have gradually increased (die refers to the individual integrated circuit component or chip prior to it being packaged). For example, it is projected that a memory device having a capacity of one gigabyte will require a die size of 30 mm per edge. For known photolithography tools, the die size is dependant on the image field size of the tools' optics. The dual constraints of higher resolution and constant, if not increasing, field sizes have made the design and manufacture of reduction lenses, as used by step and repeat cameras, ever more difficult and costly. It is anticipated that in the near future, it may not be economically feasible to produce large field lenses capable of resolving the 0.25 micron features that will be required for future generations of integrated circuits.
One known approach that addresses the dual constraints discussed above is embodied by the Perkin-Elmer Micrascan.TM. system. This system combines the step and repeat and scanning techniques and uses primarily reflective optics in a `step and scan` fashion to achieve a 4 to 1 reduction ratio. `Step and scan` refers to a technique where individual exposure fields are scanned past an image field of a reticle created by an illumination system. When the scan is complete, the system steps to the next field to be scanned. In the Micrascan system, the reticle will be positioned on a reticle stage and the wafer or mask will be positioned on a wafer stage. Since a 4 to 1 reduction is occurring, the reticle stage will be moving at a speed 4 times as great as the wafer stage. The optics in the Micrascan system are designed to produce a scanned field size of 20.times.32.5 mm when utilizing 6.times.6 inch reticles. A well corrected arcuate field that spans 20 mm on a first fixed axis (the width of an exposure field) is scanned in a second axis to produce an effective image length of up to 50 mm (limited primarily by available stage travel). This approach removes the field size constraints on one axis only. The Micrascan system is described in detail in an article entitled "Step and scan: A systems overview of a new lithography tool", authored by Jere D. Buckley and Charles Karatzas, appearing in SPIE Vol. 1088 Optical/Laser Microlithography II Pages 424-433 (1989).
A second known system is described in U.S. Pat. No. 4,924,257, Jain, entitled "Scan and Repeat High Resolution Projection Lithography System", herein incorporated by reference. The system in the Jain reference provides for extending the field size along two axis. The system in the Jain reference utilizes a hexagonal shaped image field that is scanned in a manner to create complementary exposures in an overlap region between adjacent scans. Utilizing this scanned overlapping technique, the effective field image is created along both axis.
The system in Jain relies on the synchronization of the reticle and wafer stages to insure that the proper image field is presented to the wafer. Using this technique, stage synchronization must be accurate well below 0.1 microns in order to accommodate anticipated feature sizes. It would be desirable to provide a scanning lithography system where the proper image field is presented to the wafer by means where corrections may take place in the optical path. Such a system would have the benefits of insuring accuracy while relaxing stringent stage synchronization requirements.
A third known system described in U.S. Pat. No. 4,879,605, Warkentin et al., entitled "Rasterization System Utilizing an Overlay of Bit-Mapped Low Address Resolution Databases", assigned to assignee of the present invention, provides for overlapped multiple-pass printing of a circuit pattern. In the Warkentin reference, a high address resolution database representing a high address resolution pattern is converted into a plurality of low address resolution databases. Multiple-passes utilizing the low address resolution databases are used to print the low address resolution databases resulting in a high resolution printed pattern. The method described in the Warkentin reference provides for the reduction of butting, critical dimension and placement errors.
It is an object of the present invention to develop a microlithography system that scans using small field refractive optics and which is capable of extending the field size in both axes. Small field refractive optics are desirable because of their suitability for high Numerical Aperture (NA) lenses (which provides for greater resolution). It is desirable to extend the field size along both axes to enable larger and more practical die sizes and shapes. It is a further object of the present invention to provide a highly accurate microlithography tool that will precisely place a reticle image field on a wafer through means found in the optical path.